Cryptology ePrint Archive: Report 2007/332

Abstract: It is well known that universally composable multiparty computation
cannot, in general, be achieved in the standard model without setup
assumptions when the adversary can corrupt an arbitrary number of
players. One way to get around this problem is by having a
\emph{trusted third party} generate some global setup such as a
\emph{common reference string (CRS)} or a \emph{public key
infrastructure (PKI)}. The recent work of Katz shows that we may
instead rely on physical assumptions, and in particular
\emph{tamper-proof hardware tokens}. In this paper, we consider a similar but \emph{strictly weaker}
physical assumption. We assume that a player (Alice) can
\emph{partially isolate} another player (Bob) for a brief portion of
the computation and prevent Bob from communicating more than some limited number of bits with the environment.
For example, isolation might be achieved by asking Bob to put his functionality on a tamper-proof hardware token and assuming
that Alice can prevent this token from communicating to the outside world.
Alternatively, Alice may interact with Bob directly but in a special office which she administers and where there are no high-bandwidth
communication channels to the outside world. We show that, under \emph{standard} cryptographic assumptions, such physical setup can
be used to UC-realize any two party and multiparty computation in
the presence of an active and \emph{adaptive} adversary corrupting
any number of players. We also consider an alternative scenario, in
which there are some trusted third parties but no single such party
is trusted by all of the players. This compromise allows us to
significantly limit the use of the physical set-up and hence might
be preferred in practice.